Frequency modulated optical spectrometer

ABSTRACT

An optical spectrometer is adapted to provide differential wavelength modulation of a spectral signal by vibrating a photosensitive cell in the focal plane of the spectrograph. A synchronous detector is employed to demodulate the output signal of the photosensitive cell, and thereby provide to a recording device a signal that is the derivative of the spectral signal.

United States- Patent [72] Inventors (Irya: E. Kollzller [56] ReferencesCited Nil 1) TATE P T N mbhnmkessm' 3 041 459 6/1362 0 8 J S A E TS250/226 [211 App! 823,361 reene, r. [22] Filed May 9, 1969 OTHERREFERENCES [45] Patented May 18,1971 Principles of Self-ModulatingDerivative Optical Spec- [73] Assignee California Institute ofTechnology troscopy Bonfiglioli & Brovetto; APPLIED OPTICS; Vol 3Pasadena, Calif. #12; Dec. 1964; pg. 1417-1424 Primary Examiner-RonaldL. Wibert 4 Assistant Examiner- Vincent P. McGraw 54 FREQUENCY MODULATEDOPTICAL Attorneys-Samuel Lindenberg and Arthur F [6111011 SPECTROMETERll chlmsznrawing Figs ABSTRACT: An optical spectrometer is adapted toprovide [52] US. Cl 356/74, differential wavelength modulation of aspectral signal by 250/226, 356/97, 356/98 vibrating a photosensitivecell in the focal plane of the spec- [5 l] Int. Cl G0lj 3/00, trograph.A synchronous detector is employed to demodulate G01 j 3/42 the outputsignal of the photosensitive cell, and thereby pro- [50] Field of Search250/226; vide to a recording device a signal that is the derivative ofthe 356/74; 350/101 spectral signal.

14 LIGHT SAMPLE s o u R c E I I2 IO ll 13 RECORDING V '5 Device A nLOCK- m DEMODULATOR AMP ls REFERENCE oselLLAroa PATENTEB MAY 1 8 I97! I3; 578.866

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LIGHT SOURCE SAMPLE IO N RECORDING DEVICE AM. DEMODULATOR REFERENCEOSCILLATOR BRYAN E. KOHLER ALAN s, DUB I N INVIiN'I'UIcS ATTORNEYSFREQUENCY MODULATED OPTICAL SPECTROMETER BACKGROUND OF THE INVENTIONThis invention relates to analytical spectrometers, and in particular toan improved method and apparatus for modulating the spectral signalthereof.

In the past, it has been recognized that spectrum analysis requires veryhigh stability of radiation source, radiation sensitive cell andamplifier. Various techniques have been devised to improve performanceof spectrometers, such as the addition of a chopper in the radiationbeam path. However, a chopper is generally useful only in eliminatingdrifts in the amplifier, and in rejecting noise developed in thesensitive cell and the amplifier. 5 Any improved signal-to-noise ratioachieved is at the expense of radiant power received by the cell sincethe beam is chopped off half the time.

Anotherrnore recent technique described by I. Balslev in PhysicalReview,Vol. 143( I966), at pages 640 and 641 consists of vibrating the exitslit of the spectrometer at a reference frequency in order to measurethe derivative of absorption with respect to wavelength, andsynchronously detecting the modulated signal at the reference frequency.This allows the direct detection ofsmall changes in absorption whilestudying small spectral details masked by continuous absorption, orwhile looking for extremely weak absorptions. The advantage 'over achopper is that there is no signal at the reference frequency unlessthere is an absorbing substance present in the beam path to produce anonlinear response in the cell. In

that manner, the signal-to-noise ratio of the spectrometer is improved.

Another technique after that by I. Balslev reported by A. Gilgore, etal., in Review of Scientific Instruments Vol. 38 (1967) at pages 1535and 1536, employs a rotating quartz plate to cyclically deflect thespectrum by inclining the plate at an angle 6 to the axis of rotation.Thus, the derivative of absorption with respect to wavelength ismeasured by vibrating the spectrum on the exit slit instead of vibratingthe slit.

SUMMARY OF THE INVENTION In accordance with the present invention, aspectrometer is adapted to produce frequency modulation of the spectralsignal by vibrating a sensitive cell in'the focal plane thereof. A

reference signal provided to vibrate .the cell is employed tosynchronize a detector. In that manner,.the derivative of the spectralsignal is provided for recording. The amplitude of vibration maybereadily adjusted for optimum instrument performance.

Although features of the invention to be protected are set forth withparticularity in the appended claims, the invention will best beunderstood from the following description of the illustrative embodimentwith reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates schematically apreferred embodiment in a single beam absorption spectrometer.

FIG. 2 illustrates in a simplified spectrogram, absorption and thederivative of absorption for a hypothetical absorbing substance presentin the beam path of the spectrometer of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS which directs diffracted lightinto a second mirror 16. The second mirror 16 reflects the diffractedlight onto a focal plane P-P where in lieu of a photographic plate, aphotosensitive cell 17 is connected to an armature 18 of anelectromechanical transducer 19 for vibration in the focal plane inresponse to an alternating signal applied thereto. The alternatingsignal is generated by a reference oscillator 20 and amplified by asuitable amplifier 21, such as an audio amplifier.

To clarify the exact role of the present invention, consider first theoperation of a conventional spectrometer having a stationary slit whichis not unlike the present one if the cell is held stationary.

The light within the spectrograph 13 is dispersed by the diffractiongrating 15 into its component wavelengths such that with the cell 17stationary, light that impinges on it at any given time is nearlymonochromatic i.e., the exciting light falling on the cell 17 is of asingle wavelength. Upon rotating the grating 15 about anaxis 22 (byconventional means not shown) the wavelength of the light that impingeson the stationary cell 17 is continually varied.

Due to the absorption characteristics of the sample 10, the intensity ofthe exciting light impinging on the cell 17 at any given time producesan electrical signal of proportional amplitude. This signal can berecorded on a standard device, such as an oscillograph or oscilloscope.If the grating rotates at a uniform rate, the change in the wavelengthof the exciting light with time is likewise uniform, and the recorderplots a continuous wave curve describing the intensity of the excitinglight as a function of its wavelength as illustrated by graph A of FIG.2.

Often absorption spectroscopy involves measuring'weak absorptions bydetecting small changes in the intensity of the monochromatic componentsof the irradiating light. Since the system which includes a detector andamplifier are apt to produce fluxuations with time (commonly referredtojas instrument noise), the task of measuring weak absorptions is mademore difficult, and sometimes impossible, however small thesefluxuations may be since they are dutifully recorded, thereby making lowamplitude peaks of the spectral curve produced by weak absorptions.Therefore, if the absorption peaks in the spectrogram are only as largein magnitude as the fluxuations characteristic of noise, the method ofdetecting absorption by simply rotating the diffraction grating 15 isnot adequate. In other words, providing a stationary photosensitive cell17 in the focal plane of the spectrograph 13 in lieu of a slit isscarcely more effective than providing a slit of the same dimensionswith the photosensitive cell 17 outside the spectrograph 13.

To improve the sensitivity of the system, the cell 17 is vibrated (movedback and forth) in the focal plane P-P of the spectrograph with harmonicmotion. The frequency of vibration is determined by the referenceoscillator 20 which is a conventional variable oscillator designed toproduce a stable frequency set by a dial 23. The amplitude of vibrationis then adjusted through the'amplifier 21 by setting of a dial 24. Usinga conventional audio amplifier and electromechanical transducer, theamplitude of vibration may be set by simply adjusting the volume controlof the amplifier.

The modulated signal generated by the vibrating photosensitive cell 17is amplified by a conventional lockin amplifier 25 and demodulated by aconventional demodulator 26 -to transmit to a recording device 27 asignal which is the derivative of absorption with respect to wavelength.To accomplish that, the reference frequency employed to vibrate the cell17 in the focal plane of the spectrograph 13 is applied to the lockinamplifier 25. Alternatively, the lockin amplifier 25 may be aconventional amplifier tuned to the frequency of the referenceoscillator 20.

To assist in understanding how the derivative of absorption with respectto wavelength is derived by vibrating the cell 17, consider the typicalspectral signal recorded in graph A of FIG. 2. That spectral signal isderived by a conventional signal-beam spectrometer with a stationaryslit in the position of cell 17 as the diffraction grating 15 is rotatedto sweep the component wavelengths before the slit. It should be notedthat the same spectral signal would be derived if the photosensitivecell 17 were-moved across the focal plane without vibration, therebymeasuring the intensity of light as it is moved through the points ofconvergence of the component wavelengths in the focal plane. It shouldalso be noted that the time required to sweep or scan through allcomponent wavelengths is generally in the order of minutes.

If the photosensitive cell 17 is vibrated with an amplitude equal toapproximately one-half the width of the natural spectral lines on thefocal plane P-P, the spectral signal is modulated. Upon synchronousdemodulation, the derivative of the spectral signal (graph A) isproduced as shown by the graph B. Each peak" of the spectral signal isthus represented by a differentiated wave with zero-crossover at thecenter of the peak." in other words, given the frequency of vibrationml, the cell 17 secs light of wavelength A -l-Ak cos But the amplitudeof the resulting alternating signal remains zero unless the intensity ofthe light varies along the very short distance of vibration as the totalspectrum is scanned. The greater the change in intensity for a given AAthe greater the amplitude of the modulated signal. The result is an ACcarrier signal at the frequency m! that is amplitude modulated by thederivative of the spectral signal. Noise rejection then becomes possibleby amplifying the amplitude modulated signal at the frequency out usingstandard electronic techniques for discriminating against any signals offrequencies other than wt. An output signal for the recording device 27is then derived by standard amplitude demodulation techniques.

The vibration amplitude for the cell 17 is adjusted for optimuminstrument performance. For sharp high intensity spectral lines, theamplitude of vibration may be adjusted to substantially less than halfthe line width, a line width being a peak centered about a givenwavelength by which the spectral line is identified. For low intensityspectral lines (peaks) not well defined, the amplitude may be increasedto approximately half the spectral width. The frequency of vibration mayalso be adjusted for optimum results. In general, the higher spectrallines require lower frequencies since the amplitude modulation beingachieved is a function of the rate of change of light intensity. Properadjustment of the spectrometer will therefore frequently require anadjustment in both amplitude and frequency. ln that regard, it should benoted that while the vibrating frequency is in the order of kilohertz,the time required to scan one spectrum is in the order of minutes.

The greatly improved signal-to-noise ratio provided by the presentinvention is due to various contributing factors which are for the firsttime brought together to optimize the technique described by l. Balslevof measuring the derivative of absorption with respect to wavelength. Asignificant factor is that, by substituting the photosensitive cell 17for the exit slit, substantially all of the photosensitive cell surfacereceives light at all times, thereby virtually eliminating dark current"from the cell. For instance, if less than percent of the photosensitivesurface of the cell receives a light beam at any given time, the darkcurrent" from the other 90 percent may mask the current produced by thelight beam if the light beam is weak. Another advantage is that theheight of the cell may be readily reduced, while maintaining the widthas before, to improve resolution at no loss in the signal-to-noiseratio.

To further optimize the signal-to-noise ratio, and thereby maximizesensitivity (resolution) of the instrument, it is desirable to be ableto vary the amplitude and frequency of vibration. This is greatlyfacilitated by substituting the photosensitive cell 17 for the exit slitand vibrating the cell. Since less mass is involved than in a vibratingslit arrangement, the range of vibrating frequencies is greatlyincreased. Moreover, less power is required to vibrate the cell andthere is less of a problem of isolating the rest of the system from thevibration, such as the rotating diffraction grating.

Still another advantage is elimination of the need for an exit slit bycombining the functions of modulating and sensing the spectral signal.The photosensitive cell 17 substituted for the exit slit may be producedto the dimensions desired (about 5 to 500 microns in width and up to 1cm in height) by employing standard techniques which have been developedin the field of semiconductive devices. For IR spectroscopy, a suitablelR sensitive cell or bolometer may be readily produced by the sametechniques. In each case, the mass of a photosensitive cell produced inthat manner would not add significantly to the mass of theelectromechanical transducer employed to vibrate it. For instance, ifthe driver of a loud speaker having a flat frequency response for therange of vibrating frequencies desired is selected for the transducer,connecting the photosensitive cell 17 thereto would not significantlyalter its frequency response characteristics. For higher frequencies ofvibration, a piezoelectric crystal driver would be preferred.

The rotating disc arrangement of the prior art referred to hereinbeforeis also not as simple and inexpensive as substituting the photosensitivecell for the slit and vibrating the cell. First there is the problem offinding transparent material that is refractive for the disc. Then thereis the problem of providing an adjustment in the amplitude and frequencyof modulation. It can not be done electronically; amplitude changesrequire a change in the angle between the axis of the disc and the axisof the rotating shaft on which it is mounted. Moreover, very highfrequencies are not possible as with a vibrating cell.

Other advantages of the present invention over the rotating discarrangement are that the present invention will readily work in vacuumultraviolet spectroscopy and can be used with a polarized light source.A rotating disc would produce spurious signals in response to polarizedlight.

While in the foregoing, a single embodiment has been described in somedetail, other embodiments will readily occur to those skilled in the artwithout departing from the spirit of the invention. It is thereforeintended that the present invention be limited in scope only by theterms of the following claims.

We claim:

1. In a spectrometer having means for splitting radiant energy accordingto frequency and for focusing all parts of a large frequency range in agiven focal plane to form a spectrum within an opaque enclosure, theimprovement comprising a source of a stable alternating signal and meansresponsive to said signal for vibrating a spectral beam sensitive cellat a fixed station within said enclosure in said focal plane while saidspectrum is being scanned past said fixed station in said plane toproduce as an output signal of said cell an alternating signal at thefrequency of vibration of said cell, said output signal having at anygiven time an amplitude proportional to the change of intensity ofradiation being sensed by said cell as said cell is vibrated through onecomplete cycle.

2. The improvement of claim 1 including means for adjusting theamplitude of said alternating signal to adjust the amplitude ofvibration of said cell.

3. The improvement of claim 2 including means for adjusting thefrequency of said alternating signal to adjust the frequency ofvibration of said cell.

4. In a spectrometer, the improvement comprising:

an opaque enclosure having a slit for receiving radiant enermeans withinsaid enclosure for dispersing and focusing all parts of a range offrequencies of said energy in a given focal plane to form a spectrum; acell within said enclosure, said cell being sensitive to said energy atall frequencies within said range of frequencies;

means for vibrating at a stable frequency said sensitive cell at a fixedstation in said focal plane while said spectrum is being scanned pastsaid fixed station in said plane, thereby producing an alternatingsignal at said frequency, the amplitude of said signal at any given timebeing proportional to the change in intensity of radiation energy beingsensed during one cycle of vibration; and

means for demodulating said amplitude modulated signal.

5. [n a spectrometer the combination comprising:

an opaque enclosure having only an entry slit for receiving radiantenergy from a source, and no exit slit;

means within said enclosure for dispersing a radiant beam from saidsource into spectral beams of a given spectrum;

a cell sensitive to all spectral beams of said spectrum for producing asignal in proportion to the intensity of radiation being received at anygiven time;

means for scanning said spectral beams within said enclosure in a givendirection with said cell; and

means for vibrating said cell at a stable frequency, and in a directionparallel to said given direction of scanning said spectral beams, tomodulate said signal produced by said cell into an alternating signal atsaid frequency of vibration with an amplitude of any given cycleproportional to the change of intensity of radiation being received bysaid cell as said cell is vibrated through one cycle.

6. The combination of claim 5 including means for adjusting theamplitude of vibration of said cell.

7. The combination of claim 6 including means for adjusting thefrequency of vibration of said cell.

8. The combination of claim 7 wherein said means for vibrating said cellis responsive to an alternating signal, said means for adjustingamplitude of vibration comprises means for adjusting the amplitude ofsaid alternating signal, and said means'for adjusting frequency ofvibration comprises means for adjusting the frequency of saidalternating signal.

9. In an instrument for spectroscopy of the type normally having anopaque enclosure housing a dispersing system, said enclosure having anexit slit through which components of a spectrum are caused to pass insequence for detection by a sensitive cell while scanning said spectrumin a given direction, the improvement comprising:

means for mounting said cell within said enclosure at a station normallyoccupied by said exit slit; and

means for vibrating said cell in place at a stable frequency in adirection parallel to said given direction of scanning said spectrum.

10. The improvement of claim 9 including means for adjusting theamplitude of vibration of said cell.

11. The improvement of claim 10 including means for adjusting thefrequency of vibration of said cell.

1. In a spectrometer having means for splitting radiant energy accordingto frequency and for focusing all parts Of a large frequency range in agiven focal plane to form a spectrum within an opaque enclosure, theimprovement comprising a source of a stable alternating signal and meansresponsive to said signal for vibrating a spectral beam sensitive cellat a fixed station within said enclosure in said focal plane while saidspectrum is being scanned past said fixed station in said plane toproduce as an output signal of said cell an alternating signal at thefrequency of vibration of said cell, said output signal having at anygiven time an amplitude proportional to the change of intensity ofradiation being sensed by said cell as said cell is vibrated through onecomplete cycle.
 2. The improvement of claim 1 including means foradjusting the amplitude of said alternating signal to adjust theamplitude of vibration of said cell.
 3. The improvement of claim 2including means for adjusting the frequency of said alternating signalto adjust the frequency of vibration of said cell.
 4. In a spectrometer,the improvement comprising: an opaque enclosure having a slit forreceiving radiant energy; means within said enclosure for dispersing andfocusing all parts of a range of frequencies of said energy in a givenfocal plane to form a spectrum; a cell within said enclosure, said cellbeing sensitive to said energy at all frequencies within said range offrequencies; means for vibrating at a stable frequency said sensitivecell at a fixed station in said focal plane while said spectrum is beingscanned past said fixed station in said plane, thereby producing analternating signal at said frequency, the amplitude of said signal atany given time being proportional to the change in intensity ofradiation energy being sensed during one cycle of vibration; and meansfor demodulating said amplitude modulated signal.
 5. In a spectrometerthe combination comprising: an opaque enclosure having only an entryslit for receiving radiant energy from a source, and no exit slit; meanswithin said enclosure for dispersing a radiant beam from said sourceinto spectral beams of a given spectrum; a cell sensitive to allspectral beams of said spectrum for producing a signal in proportion tothe intensity of radiation being received at any given time; means forscanning said spectral beams within said enclosure in a given directionwith said cell; and means for vibrating said cell at a stable frequency,and in a direction parallel to said given direction of scanning saidspectral beams, to modulate said signal produced by said cell into analternating signal at said frequency of vibration with an amplitude ofany given cycle proportional to the change of intensity of radiationbeing received by said cell as said cell is vibrated through one cycle.6. The combination of claim 5 including means for adjusting theamplitude of vibration of said cell.
 7. The combination of claim 6including means for adjusting the frequency of vibration of said cell.8. The combination of claim 7 wherein said means for vibrating said cellis responsive to an alternating signal, said means for adjustingamplitude of vibration comprises means for adjusting the amplitude ofsaid alternating signal, and said means for adjusting frequency ofvibration comprises means for adjusting the frequency of saidalternating signal.
 9. In an instrument for spectroscopy of the typenormally having an opaque enclosure housing a dispersing system, saidenclosure having an exit slit through which components of a spectrum arecaused to pass in sequence for detection by a sensitive cell whilescanning said spectrum in a given direction, the improvement comprising:means for mounting said cell within said enclosure at a station normallyoccupied by said exit slit; and means for vibrating said cell in placeat a stable frequency in a direction parallel to said given direction ofscanning said spectrum.
 10. The improvement of claim 9 including meansfor adjusting the amplItude of vibration of said cell.
 11. Theimprovement of claim 10 including means for adjusting the frequency ofvibration of said cell.